It is conventionally known that if light from a high-luminance subject such as the sun enters an imaging optical system, it is reflected by the surface of a lens, the inner surface of a barrel, etc. and becomes stray light. Because such stray light becomes a cause to form unintended images (ghosts) in an imaging screen, technologies for preventing stray light have been proposed in order to reduce the ghosts.
For example, disclosed in Japanese Unexamined Patent Application No. H10-62669 is a technology to reduce ghosts by varying the operating size of an aperture corresponding to an effective luminous flux that varies based on an aperture value (Patent Document 1). Also, proposed in Japanese Unexamined Patent Application No. H8-334725 is a technology to remove harmful light by restricting a diaphragm to a smaller aperture value than its open value. Furthermore, proposed is a method to suppress ghosts by reducing reflectance by coating the lens surface (Patent Document 2).